Dispersion and stability mechanism of Pt nanoparticles on transition-metal oxides

The heterogeneous catalysts of Pt/transition-metal oxides are typically synthesized through calcination at 500 °C, and Pt nanoparticles are uniformly and highly dispersed when hydrogen peroxide (H2O2) is applied before calcination. The influence of H2O2 on the dispersion and the stability of Pt nanoparticles on titania-incorporated fumed silica (Pt/Ti–FS) supports was examined using X-ray absorption fine structure (XAFS) measurements at the Pt L3 and Ti K edges as well as density functional theory (DFT) calculations. The local structural and chemical properties around Pt and Ti atoms of Pt/Ti–FS with and without H2O2 treatment were monitored using in-situ XAFS during heating from room temperature to 500 °C. XAFS revealed that the Pt nanoparticles of H2O2-Pt/Ti–FS are highly stable and that the Ti atoms of H2O2-Pt/Ti–FS support form into a distorted-anatase TiO2. DFT calculations showed that Pt atoms bond more stably to oxidized–TiO2 surfaces than they do to bare- and reduced–TiO2 surfaces. XAFS measurements and DFT calculations clarified that the presence of extra oxygen atoms due to the H2O2 treatment plays a critical role in the strong bonding of Pt atoms to TiO2 surfaces.


Results
Temperature-dependent XANES spectra. For this study, Pt nanoparticles were synthesized on titaniaincorporated fumed silica (Ti-FS) supports with and without H 2 O 2 treatment 9,10 , as summarized in Fig. 1. The size and the distribution of Pt nanoparticles were examined by energy dispersive spectroscopy (EDS) and TEM measurements, as shown in Fig. 2. X-ray absorption near edge structure (XANES) is sensitive to the chemical valence state as well as the geometry of the nearest neighboring atoms around a probing atom 42,43 . Temperaturedependent XANES at the Pt L 3 edge shows a dramatic change in the white line intensity during heating, as shown in Fig. 3a and b. Changes in the Pt white line are known to be directly related to Pt oxidation 11,44,45 . Jeong et al. clarified that the white line area of Pt L 3 edge directly corresponds to the coordination number of oxygen atoms bonding to Pt atoms 11 . When a Pt atom bonds with oxygen atoms, the empty state density of the Pt 5d orbitals increases because the electrons in the Pt 5d orbitals transfer to the oxygen atoms. The strong intensity of the white line and the slight shift toward a higher energy of the Pt absorption edge of both Pt/Ti-FS and H 2 O 2 -Pt/ Ti-FS at the room temperature (RT) indicate a high oxidation of Pt atoms compared to those of a Pt foil, as shown in Fig. 3a and b. The white line features strongly suggest that the Pt atoms on Ti-FS supports with no matter of the H 2 O 2 treatment at RT are oxidized to PtO x . At RT, the intensity of the Pt white line of H 2 O 2 -Pt/ Ti-FS is substantially stronger than that of Pt/Ti-FS, as shown in Fig. 3a and b, respectively. This indicates that the mean oxidation state of Pt atoms of H 2 O 2 -Pt/Ti-FS is higher than that of Pt atoms of Pt/Ti-FS, which is attributed to the H 2 O 2 treatment. This agrees with the previous reports which showed an oxidizing agent of H 2 O 2 to metals [31][32][33]37 . The intensity increase of the white line might suggest the change of the Pt precursor from [Pt(NH 3     Temperature-dependent local structural properties. EXAFS can be used to quantitatively determine the local structural properties around a selected species atom of compounds [46][47][48] . After atomic background function was determined using the AUTOBK code 49 , EXAFS data was extracted from XAFS and Fourier transformed to the r-space, as shown in Fig. 4. EXAFS was quantitatively analyzed with the IFEFFIT package 50 using standard analysis procedures 48,51 . The peak positions of EXAFS data correspond to the atomic shell distances from www.nature.com/scientificreports/ a probing atom. The peak positions of the EXAFS data are approximately 0.3 Å shorter than the true distances of atomic pairs because the phase shift of back-scattered photoelectrons by neighboring atoms has not yet to be considered. At the Pt L 3 edge, the temperature-dependent EXAFS of Pt/Ti-FS with and without H 2 O 2 treatment reveals that the local structure around Pt atoms is significantly changed by heating under a H 2 environment, as shown in Fig. 4a Fig. 4c A and C. These peaks respectively correspond to the Ti-O and Ti-Ti pairs of a TiO 2 structure 52,53 . For the Ti-FS at RT, the intensity of the first peak at ~ 1.7 Å increases and the shape of the second peak at ~ 2.5 Å changes, compared with those at RT c . This corresponds to the XANES spectra, as shown in Fig. 3c A and C. The distance of the Ti-Ti pairs is expanded due to the H 2 O 2 treatment, compared to that of Ti-FS at RT c , as shown in Fig Quantitative analysis of local structural properties. Using the standard fitting procedures, EXAFS data in the r-space were fitted to EXAFS theoretical calculations with different structural models at the Pt L 3 and Ti K edges 48,51 . The structural models of EXAFS theoretical calculations were designed based on the measured XANES and EXAFS data. PtO x and Pt foil structures were initially modeled for the low and high temperatures of both Pt/Ti-FS and H 2 O 2 -Pt/Ti-FS, respectively. Pt precursors likely have the chemical formulas of [Pt(NH 3 ) 4 ] (NO 3 ) 2 and [Pt(NH 3 ) 4 (OH) 2 ](NO 3 ) 2 for the Pt/Ti-FS and H 2 O 2 -Pt/Ti-FS, respectively, at RT. EXAFS cannot distinguish between O and N atoms and cannot detect H atoms due to the resolution limit. Furthermore, the local structure around Pt atoms are substantially changed when heated, as shown in Fig. 4. Thus, we chose a PtO x structural model for the EXAFS data fits at low temperatures. At intermediate temperatures, a mixture structure of PtO x and Pt foil was used to fit EXAFS data. For the EXAFS data fitting of the Ti K edge, structural models were selected based on the XANES and EXAFS data of Ti-FS and Pt/Ti-FS. In the fitting of the EXAFS data of H 2 O 2 -Pt/Ti-FS, a distorted-anatase TiO 2 structure was used for the EXAFS theoretical calculations. EXAFS theoretical calculations were done using the FEFF8 code 54 , and EXAFS data was fitted using the IFEFFIT package 50 . In the fittings, the distance, the coordination number, and the Debye-Waller factors ( σ 2 , including thermal vibration and static disorder) of each atomic shell were varied. Only single-scattered paths were included in the fittings; this decision was made because, due to the particle size and the structural disorder, the EXAFS signal of a multiple-scattered path of nanoparticles is much weaker than that of a single-scattered path. A k-weight fit was used to reduce the correlation between σ 2 and coordination number 53 . Figure 5 shows representative EXAFS data and the best fits. The quantitative structural properties around the Pt and Ti atoms of Pt/Ti-FS and H 2 O 2 -Pt/Ti-FS were obtained from the goodness fits of the EXAFS data. The results of the best fits are summarized in Tables 1-3 Fig. 4a. N is the coordination number, d is the distance, and σ 2 is the Debye-Waller factor of atomic pairs. S 0 2 = 0.86 was determined by fitting the EXAFS data of a Pt foil and used in the other fittings. (1) RT 5.0(6) 2.021(6) 0.003 (1) 5(1) 2.856(9) 0.009 (1) 5(1) 2 55 . The role of oxygen atoms to separate metal atoms in atomic scale was also observed in Pd nanoparticles 56 . Our study suggests that the oxygen atoms around Pt atoms at the initial stage of the synthesis process play a decisive role in a high dispersion of Pt nanoparticles; meanwhile, they at the interface of Pt nanoparticles and Ti-FS supports assist a strong bond between Pt atoms and TiO 2 supports when heated to a high temperature. When heated above 250 °C, the coordination number of Pt atoms of Pt/Ti-FS is gradually increased to be ~ 10 at 500 °C, whereas that of H 2 O 2 -Pt/Ti-FS shows a lack of changes in the temperature range of 250-500 °C. This result strongly implies that, at high temperatures, the Pt atoms of Pt/Ti-FS and H 2 O 2 -Pt/Ti-FS move to become lumpy and are pinned on the supports, respectively. A large σ 2 value of the Pt-Pt pairs of Pt/Ti-FS indicates a less stable structure of Pt nanoparticles than that of H 2 O 2 -Pt/Ti-FS, particularly at RT c . When cooled down to RT from 500 °C, the local structural properties around Pt atoms of both Pt/Ti-FS and H 2 O 2 -Pt/Ti-FS are nearly the same as those at 500 °C, except for the σ 2 values of Pt-Pt pairs due to the thermal effect. Jeong and co-workers demonstrated using EXAFS measurements that the Pt nanoparticles of H 2 O 2 -Pt/Ti-FS likely have a pancake shape on TiO 2 supports 11 . In this case, Pt nanoparticles stably bond to the supports with a high catalysis efficiency 9 . At RT c , the bond lengths of the Pt-Pt pairs of both Pt/Ti-FS and H 2 O 2 -Pt/Ti-FS are shorter than that of a Pt foil due to the effects of nanoparticle boundaries with dangling bonds 57 .
The dispersion and the structural stability of Pt nanoparticles can be affected by supports. TEM images show that lump Pt particles form on FS supports, while small and uniform Pt nanoparticles are spread over at Ti-FS supports, as shown in Fig. 2e-g. This agrees well with previous observations 9, 10 . Previous EXAFS measurements revealed that most of the Pt nanoparticles bond with the Ti atoms of the Ti-FS supports 11 . This indicates that most of the Pt atoms choose TiO x complexes rather than FS as supports, when the uniform mixture of the Pt precursor and the calcined Ti-FS powder is heated up to 500 °C. It is noted that the weight ratio of the Ti precursor and the FS was 2:1. The EXAFS data of Ti-FS and Pt/Ti-FS with and without H 2 O 2 treatment at the Ti K edge were also analyzed in the same manner as the EXAFS data analysis at the Pt L 3 edge, and the best fit results are summarized in Table 3. The best fit of EXAFS data at the Ti K edge suggests that the Ti atoms of Ti-FS form Ti-O complexes in the temperature range of RT -500 °C. When an H 2 O 2 treatment is applied on the Ti-FS, the local structure around the Ti atoms is substantially changed, compared to that of the Ti-FS at RT c ; however, TiO x do not form into a crystalline structure, as shown in   [62][63][64] . It has dimensions of 11.4 Å × 11.4 Å × 20 Å containing a vacuum region of about 15 Å to avoid interactions between the periodically-repeated slabs and to separate the slabs. Figure 6a and b show a part of the TiO 2 (101) surface of the supercell slab. In the calculations, the atoms of the bottom two layers-i.e., the lower half of the slab-are fixed at the original positions of anatase TiO 2 , and the rest of the atoms are allowed to freely move their positions to minimize the total energy of the system. A Pt atom is added on three different TiO 2 (101) surfaces, such as bare-TiO 2 , oxidized-TiO 2 , and reduced-TiO 2 , as shown in Fig. 6c-e, respectively. Oxidized-and reduced-TiO 2 (101) surfaces are generated by adding one O atom on the top of the 5cTi atom and by removing the bridging O atom (2cO), respectively, as shown in Fig. 6d and e, respectively 65 . Pt adatoms are initially placed in the middle of the hexagons of the TiO 2 (101) surfaces in Fig. 6c1, d1, and e1. After DFT calculations, the final locations of the Pt atoms are shown in Fig. 6c2, c3, d2, d3, and e2, e3. Before the DFT calculations, we first examined the cutoff energy of plane waves in the range of 300-700 eV and the k-point grid using the Monkhorst-Pack method to reduce the computation time and obtain sufficient precision. The ultra-soft pseudopotentials with a cutoff energy of 550 eV, 3 × 3 × 2 k-point grids, and Perdew-Burke-Ernzerhof (PBE) functional of the generalized gradient approximation (GGA) are used 66 . The top layers of the slab including Pt adatoms and overlayers are relaxed until the force is less than 0.1 eVÅ -1 in the Cartesian coordinates. The calculation is terminated when the total energy difference between the present and final calculations converges to less than 5 × 10 -5 eV per atom. The binding energy of a Pt atom on TiO 2 is defined as △E = E Pt/TiO2 − E TiO2 − E Pt , where E Pt/TiO2 is the total energy of a Pt atom on TiO 2 ; E TiO2 is the total energy of a TiO 2 slab for bare-TiO 2 , oxidized-TiO 2 , and reduced-TiO 2 cases; and E Pt is the total energy of a single Pt atom. The DFT calculations show △E = − 2.145 eV, − 6.515 eV, and − 4.455 eV for bare TiO 2 , oxidized TiO 2 , and reduced TiO 2 , respectively. This result indicates that Pt atoms more strongly bond to oxidized or reduced TiO 2 than they do to bare TiO 2 . A strong bond of Pt atoms to reduced-TiO 2 surface corresponds to a strong metal-support interaction (SMSI). Previous studies have shown that the catalysis efficiency of noble-metal on transition-metaloxide supports decreases by reduction at a high temperature due to an SMSI [67][68][69][70] . The DFT calculations show that the bond of Pt atoms to oxidized-TiO 2 is more stable than the bond of Pt atoms to the others, as shown in Fig. 6. In this study, we first report a strong bond of Pt atoms to oxidized-TiO 2 surfaces, which is the origin of the high dispersion and high stability of Pt atoms on Ti-FS supports, based on the EXAFS measurements and the DFT calculations.

Wavelet-transformed EXAFS analysis.
The DFT calculations suggest that there are Pt-O bonds at the interfaces of Pt/TiO 2 during the processes of the H 2 O 2 treatment and the heating above 250 °C. Since Fouriertransformed (FT) EXAFS analysis does not show a distinguishable feature of Pt-O pairs, as shown in Fig. 5, we performed the wavelet-transformed (WT) EXAFS analysis which has structural information in the k-space as well as in the r-space. Figure 7 shows the WT-EXAFS images of the Pt foil and the H 2 O 2 -Pt/Ti-FS at different temperatures. The EXAFS signal of the Pt atoms of the Pt foil in the k-and r-spaces is quite different from that   Fig. 7a  The mean diameters of the Pt nanoparticles of H 2 O 2 -Pt/Ti-FS and Pt/Ti-FS are estimated to be ~ 11 Å and ~ 22 Å for the Pt coordination numbers of 7.9 and 9.6, respectively, using a hemispherical model 11 . The mean diameters suggest that an H 2 O 2 treatment prevents the agglomeration of Pt atoms on TiO 2 at high temperatures. The SMSI effects of noble metal/transition-metal oxides have been observed on several different systems [67][68][69][70] . Heterogeneous catalysts of noble-metal nanoparticles/transition-metal-oxide supports are widely used for practical applications [24][25][26][27] . With an H 2 O 2 treatment, oxygen atoms penetrate into Pt/Ti-FS and form the bond of Pt-O-Ti at the interface of Pt/TiO 2 , as shown in Fig. 6d2 and d3. The interfaces of Pt atoms and TiO 2 supports are initially somewhat unstable due to dangling bonds of the TiO 2 surface. When H 2 O 2 is applied to Pt/TiO 2 , an additional oxygen atom forms a stable Pt-O-Ti bond at the interface, thus reducing the surface energy. The additional oxygen atoms at the interface simultaneously bond with Pt and Ti atoms, and they form a Ti-O octahedron. As a result, the Pt atoms on Ti-FS supports are restricted and do not aggregate at high temperatures. A lack of change of the Pt coordination numbers of H 2 O 2 -Pt/Ti-FS in the temperature range of 200-500 °C and at RT c , as shown in Table 2, indicates that the bonds of Pt atoms to TiO 2 surfaces are considerably strong and remain constant even at 500 °C. This finding is consistent with the conversion results of the heterogeneous catalysts of H 2 O 2 -Pt/Ti-FS 9,71 .
Oxidation due to H 2 O 2 treatments has been observed in many different transition-metal systems, including Co, Zr, Mn, Ti, Zn, and Cr [30][31][32][33]35,36,40 . Imamura and co-workers suggested that H 2 O 2 molecules on a metal surface are converted into hydroxyl anions and hydroxyl radicals ( * OH) as follows 72 , H 2 O 2 + e -→ *OH + [OH] -. Previous studies have shown that an H 2 O 2 treatment changes the surface morphology of metals, assists the bonds of heterogeneous metal atoms, and causes oxidation of metal surfaces [30][31][32][33]35,36,40 . Our XAFS measurements and DFT calculations are consistent with the previous reports. When the Pt precursors are embedded into Ti-FS supports    In-situ XAFS measurements. In-situ XAFS measurements were taken of Pt/Ti-FS specimens with and without H 2 O 2 treatment at the Pt L 3 edge (11,564 eV) and the Ti K edge (4,965 eV) with a transmission mode under H 2 environment in the temperature range of room temperature (RT) -500 °C. The XAFS measurements were carried out by selecting the incident X-ray energy with a three-quarters-tuned Si(111) double crystal monochromator at the 9BM beamline of the Advanced Photon Source (APS) and at the 8C beamline of the Pohang Light Source II (PLS II). To avoid self-absorption effects, the specimen powders were ground and sieved with a sieve having a mean size of 25 μm. The powders were homogeneously mixed with a boron-nitrite powder and pressed into a disk shape with a proper thickness in a hole of a copper sample holder for the absorption edge step sizes of 0.3-0.8 at both the Pt L 3 and Ti K edges 37 . The specimens were maintained at a constant and uniform temperature during the XAFS scans.

Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.